. The objectives of the present proposal have been expanded to enable a clearer definition of structureactivity relationships, mechanism of action, binding affinity, and kinetics for HIV reverse transcriptase, and cell uptake for the oligo- and polynucleotides synthesized to date and proposed herein. Emphasis will be placed on identifying one or more potential clinical candidates, then adding sufficient value to them by the types of studies noted above to interest a major industrial partner in further development. The specific aims are as follows: 1. Continue the design and synthesis of novel purine-derived bases to enable the refinement of structure-activity relationships and test the hypotheses which have been developed during the course of this work. 2. Prepare modified oligonucleotide backbones, particularly phosphorothioate and PNA analogues, and compare their biological properties with the corresponding phosphodiester derivatives. 3. Utilize sugar modifications, especially 2'-O-methyl and 2'-O-methoxyethyl, to confer nuclease stability and facilitate oligonucleotide synthesis. 4. Utilizing UV and circular dichroism spectroscopy, MALDI-TOF, and electrospray MS and, as appropriate, NMR, characterize the physico- chemical properties of the newly synthesized compounds. 5. Append fluorescein derivatives to oligo- and polynucleotides, and examine the possibility that some of the bases used will exhibit useful fluorescence in their own right. 6. In collaboration with Dr. Jindrich Kopecek, study the cell uptake and localization of the polymers using fluorescent confocal microscopy.7. In collaboration with Dr. Robert Fisher, examine the binding affinity and kinetics of selected oligos and polymers with HIV reverse transcriptase using surface plasmon resonance (BIAcore).8. In continuing collaboration with Dr. Robert Buckheit, examine the anti- HIV activity, ability to produce resistance and mechanism of action of the synthetic oligos and polymers. 9. In continuing collaboration with Drs. Robert Sidwell and John Huffman, evaluate the anti-HCMV activity of these compounds.